To keep the years in sync with the seasons, Thai lunar years may add a day to the 7th month or repeat the 8th month. Therefore, years may have one of three lengths – 354, 355 or 384 days – yet retain a nominal length of twelve months.

354 day-long years consist of 12 "normal months", and such a year is called a "normal-month year" (Thai: (ปี) ปกติมาส, rtgs: (pi) pakatimat, [(pī) pà.kà.ti.mâːt]).

355 day-long years add an extra day to the normally 29-day-long 7th month; such a year is called an "extra-day year" (Thai: (ปี) อธิกวาร, rtgs: (pi) athikawan, Thai pronunciation:[(pīː) ʔà.tʰí.kà.waːn]).

AD 2004/2547BE Extra lunar month ended August 15

384 day-long years repeat the 30-day-long 8th month, thus keeping the month count at 12. Nevertheless, a year of 384 days is called an "extra-month year" (Thai: (ปี) อธิกมาส, rtgs: (pi) athikamat, [(pīː) ʔà.tʰí.kà.mâːt]).

The Thai lunar calendar does not mark the beginning of a new year when it starts a new 1-to-12 count, which occurs most frequently in December.

August 1 and 2, 2004. Sunday, a holiday, on the left, and Monday, observed as the compensatory day, on the right

The Thai solar calendar determines a person's legal age and the dates of secularholidays, including the civil new year and the three days of the traditional Thai New Year, which begin the next Twelve-year animal cycle. Should the holidays fall on a weekend, it also accommodates these as well as some of the principal lunar festivals with a compensatory day off (Thai: วันชดเชย, rtgs: wan chotchoei).

13 April of the solar calendar occasions the beginning of the traditional Thai New Year (Songkran) and is the day that a year assumes the name of the next animal in the twelve-year animal cycle;[1] Thai Chinese communities may observed the name-change earlier in accordance with the Chinese New Year.

In the modern Thai calendar, months (Thai: เดือน, rtgs: duean, [dɯ̄an], meaning "month" or "Lunation") are defined by lunar cycles. Successive months (or lunations) are numbered from 1 to 12 within the Thai year. As in other Buddhist calendars, these months have names that derive from Sanskrit, but for the most part are only known by Thai astrologers.[2]

Month 1, "duean ai" (เดือนอ้าย, [dɯ̄an ʔâːj]), begins the cycle of counting the months anew, most frequently in December, but does not signify the beginning of a new year.[2]ai, an archaic word in Thai but not in other dialects, means first.[3] An odd-numbered hollow month, it is 29 days long.

Buddhist Sabbaths, colloquially called วันพระ, are the New, First-quarter, Full, and Third-quarter Moon-days. These are not normally days off (วันหยุด), except for butcher, barber, and beautician shops that observe the Eight Precepts. Annual holidays and seasonal festivals collectively are called วันนักขัตฤกษ์.

Note that Loi Krathong dates are based on the Lanna (Northern Thailand) Lunar Calendar which is two months later than the Thai Lunar Calendar. Loy Krathong is actually on the second month of the Lanna calendar which is the 12th month of the central Thai calendar.

Notes:

† The Chinese New Year uses different methods of determining intercalary months, so this festival sometimes occurs a month earlier or later.

Thai orthography spells most native words phonetically, though there is no definitive system for transcription into Roman letters. Here, native Thai words are immediately followed by a vocabulary entry in this pattern:

Sanskrit loan words follow different rules [the way English grammatical rules vary for words of Greek and Latin origin ('ph-' in 'phonetic' being pronounced /f/, for example.)] Entered below in order of first appearance, these vocabulary entries are in this pattern:

Sanskrit สันสกฤต (สันสะกฺริด /san-sa-krit/)

Literally means "self-made" or "self-done", or "cultured" in a modern usage (which implies the language of cultured persons); Sanskrit alphabet, language, writing; [presumed] compound of

^J.C. Eade. The calendrical systems of mainland southeast asia. E.J. Brill, Leiden. p. 22. ISBN90-04-10437-2. According to some scholars including George Coedes, the change originally occurred at the beginning of the 5th lunar month, a few days before Songkhran.

^ abcDiller, Anthony; Preecha Juntanamalaga (2000). "Thai Time". Faculty of Asian Studies Australian National University. p. 25. Archived from the original on 2002-01-10. Retrieved 2008-05-08. (5.1) ...names would be known only by Thai astrologers (Prasert Na Nagara 1998:524).

Diller, Anthony; Preecha Juntanamalaga (December 1995). "Thai Time". International Conference on Tai Languages and Cultures, Thammasat University. Australian National University. Archived from the original on 2002-12-10. Retrieved 22 June 2008.